1. Field of Invention
This invention relates to gas analyzers for detecting low concentrations of a specific gas and is particularly directed to an improvement in such gas analyzers such that it is possible to detect low concentrations of a selected gas, carbon monoxide (CO), in the order of 5 parts per million (p.p.m.) in atmospheric air with a typical full scale capability of 130 p.p.m., over-ranging to as high as 400 p.p.m.
This invention also relates to a method of detecting low concentrations of a selected gas, such as CO.
2. Prior Art
The U.S. Pat. No. 4,163,899, entitled "Method & Apparatus for Gas Analysis" of I. G. Burough, discloses a pressure modulated infrared gas analyzer which utilizes an air pump as a pressure modulator pulsating through a sample chamber at a first frequency to produce modulation of the absorption of IR energy due to gas density changes, and modulation of the IR source intensity through said sample chamber at a second frequency to allow detection of IR source intensity changes.
This analyzer is used for the detection of CO with a full-scale sensitivity of approximately 3000 p.p.m. and resolution of approximately 100 p.p.m. which is 20 to 100 times less sensitive than the gas analyzer constructed in accordance with the teachings of this invention.
Inherent in the method and apparatus of the prior art gas analyzer is a limitation which prevents detection of levels of CO below about 50 to 100 p.p.m. With zero CO present in the sample chamber, an output equivalent to from 10 to 50 p.p.m. of CO is obtained; this may be referred to as an "offset". This offset may be due to several factors some of which are:
(1) Modulation of the IR energy due to vibration of the chamber walls,
(2) Modulation of the IR energy due to the presence in the sample chamber of gases, such as carbon dioxide, which have energy absorption bands slightly overlapping that of the CO interference filter utilized,
(3) Coupling of electrical energy from the pressure modulation source into the infrared detector circuitry (only a few microvolts is enough to produce several p.p.m. CO equivalent signal).
Suggestions have been made to improve the resolution of the prior art gas analyzer from 100 p.p.m. to 5 p.p.m. One suggestion was to use a catalytic CO removal cell after each measurement which was determined to be unacceptable for several reasons:
(a) the CO removal cell required an electro-mechanical valve to divert the sample into the removal cell. If four readings per minute are required on a continuous basis, the valve must cycle over two million times per year. By valving the sample gas containing CO through a catalytic CO removal cell, a reading of the offset may be obtained, and the offset subtracted from the reading obtained with the CO present in the sample. This procedure must be repeated each time a CO measurement is desired.
(b) a typical removal cell is constructed with a catalytic material which must be heated to provide optimum efficiency of CO removal. The energy required for heating the cell can be significant.
Another suggestion was to utilize a sample chamber which is resonant at the pressure modulation frequency, thus increasing the efficiency of the pressure modulation effect.
Another suggestion was to utilize a sample chamber with highly polished walls to act as a light guide, thus increasing the amount of modulated infrared received at the detector.
Both of these latter two suggestions were determined to be feasible but not adequate to achieve the sensitivity desired for a gas analyzer.